1. Field of the Invention
The present invention relates to a measuring apparatus, and in particular, relates to a measuring apparatus having a measurement head measuring an object, the measurement head being configured to be displaceable at least in a vertical direction.
2. Description of Related Art
Measuring apparatuses exist which perform various measurements related to a measured object by displacing a measurement head in a vertical direction (gravity direction, for example). One example of these is an image measuring apparatus capturing an image of the object placed on a stage and measures a size (such as an external dimension or a hole diameter) of the object by processing the image. The measurement head capturing the image of the object is provided with a camera (image acquirer). A position relationship between the measurement head and the object can be defined along at least three axes in X, Y, and Z directions.
In a measuring apparatus of this kind, the measurement head is a heavy object having the camera and an optical system. Therefore, a heavy load is imposed on a driver when displacing the measurement head. Particularly, in a case where the measurement head is displaced in the vertical direction (Z axis direction), in order to displace the measurement head to a desired height and to maintain the measurement head at that height, a drive mechanism such as a motor is desired with high durability and reliability. In addition, maintaining the measurement head at an arbitrary height position becomes difficult when power is off.
Japanese Patent Laid-open Publication No. 2001-356012 discloses a configuration in which a balance weight is provided to a mechanism displacing a detecting element such as a touch probe of a coordinate measuring apparatus in the Z axis direction. However, in a case where the balance weight is provided to counterbalance the measurement head (a heavy object like the image measuring apparatus), a balance weight having an equal weight to the measurement head is required. Accordingly, weight reduction of an overall apparatus becomes difficult and a space to arrange a large balance weight becomes necessary, and therefore size reduction of the apparatus becomes difficult.
Therefore, a configuration may be considered where a spring is used instead of the balance weight. In other words, a force of the measurement head attempting to descend is counteracted by an elastic force from the spring. By using the spring, the weight and size reduction of the apparatus can be achieved.
However, when using the spring, stretching of the spring (elastic force) changes based on a position of the measurement head and constant resistance force can not be obtained. Therefore, control to maintain position based on where the measurement head is positioned becomes difficult.